Cannizzaro reaction involving formaldehyde

Problem 19.16 From examination of the mechanism, can you suggest one factor that would tend to make a crossed Cannizzaro reaction involving formaldehyde take place in the particular way it does ... 

A crossed Cannizzaro reaction involving two different aldehydes may be performed if neither of the aldehydes contains a-hydrogens. Write an equation for the crossed Cannizzaro reaction of a mixture of 4-chlorobenzaldehyde and formaldehyde with concentrated potassium hydroxide solution, followed by acidification of the reaction mixture. Show all organic products. 

Dismutations of the same type, but involving two unlike aldehyde molecules, will be classed as crossed Cannizzaro reactions. The reduction of benzaldehyde to benzyl alcohol by means of formaldehyde and alkali is an example. 

The most useful examples of the Cannizzaro reaction as applied to aliphatic aldehydes are those involving /3-hydroxyaldehydes formed by aldol condensation of aldehydes which have a-hydrogen atoms or of these aldehydes with formaldehyde. Since, in these reactions, the 0-hydroxyaldehyde is not isolated, examples will be classified under the headings of the reactants which serve as the actual starting materials. 

The Cannizzaro reactionof non-enolizable aldehydes is another example of a hydride transfer reaction. It is carried out under alkaline conditions and involves not only the addition of a hydroxide ion to one aldehyde but the stabilization of the resultant acid as the anion (Scheme 3.41). Methanal (formaldehyde), which gives methanoic acid (formic acid), a relatively strong carboxylic acid, makes a good hydrogen donor in a cross-Cannizzaro reaction. 

In the Cannizzaro reaction (62), which involves self-condensation of aldehydes that have no a-hydrogen atoms, the nucleophile is represented best as a hydride ion in the Prins reaction (26), the nucleophile is an olefin in the aldol condensation (63) and reaction of methyl acetate with formaldehyde (62), the nucleophile is a methylene group a- to a... 

These primarily concern the routes involving the use of formaldehyde in transformations of the Cannizzaro reaction type in the presence of catalysts based on padnk or alkaline, earth alkali metals etc. and passing through the following intermediates ... 

Reduction. The familiar preparation of pentaerythritol from acetaldehyde and formaldehyde involves aldolization and crossed Cannizzaro reduction. In the same way, cyclohexanone reacts with 5 moles of formaldehyde to give the pentaol (3). The reaction is conducted by adding calcium oxide (1,25 moles) to a stirred mixture O OH... 

At conversions higher than 90%, the severity of the reaction has increased to a level where formose products, rather than formaldehyde, are converted. At this point, as the reaction rate in a tank reactor cannot exceed the feed rate, the rate of conversion of formaldehyde drops asymptotically to the formaldehyde feed-rate being used. This situation of extreme severity probably involves both Cannizzaro and cross-Cannizzaro conversion of products. 

The Cannizzaro i-eaction, previously discussed, involves the reduction of formaldehyde by formaldehyde. Since formaldehyde is apparently a more powerful reducing agent than most other aldehydes, this reaction can be utili2ied for the reduction of other aldehydes by a mixed Cannizzaro reac-tioii ... 

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